The present invention is directed to semiconductor devices such as transistors and the like, and more particularly is concerned with the modification of surface fields in the active regions of the devices.
The electrical field which inherently exists at the surface of an active region of a semiconductor device influences one or more of the operating characteristics of that device. For example, in an MOS field effect transistor, the field which exists at the interface of the gate oxide and the active channel region determines the threshold voltage of the device and the mobility of carriers. When the active channel region is comprised of doped silicon and the gate oxide comprises silicon dioxide, the nature of this interface is that it always has a positive charge. As a result, an n-channel MOS device typically operates in a depletion mode. It is desirable to be able to introduce a stable negative charge at the interface of the silicon and the gate oxide, to thereby reduce the positive field that exists at this interface and produce a more neutral device, or even an enhancement mode device.
Similarly, in bipolar transistors it is desirable to operate with low collector currents to thereby reduce power requirements. However, the low current gain of the transistor is affected by the recombination of carriers at the surface of the base region. This recombination is dependent upon the field which exists at this surface. If this field can be appropriately controlled, the surface recombination velocity can be influenced to bring the low current cutoff of the transistor lower.
Accordingly, it is desirable to be able to control the electric field that exists at the surface of a material in a monolithic device. In particular, it is desirable to be able to lower the net positive charge that is inherently present at the interface of a dielectric material and a semiconductor material.